This invention relates to the regeneration of fine particulate catalysts which have become temporarily deactivated by the deposition of carbonaceous material on utilization in hydrocarbon conversion processes such as catalytic cracking. More particularly, this invention is directed to an improved fluidized bed regeneration process and apparatus for a carbon-contaminated catalyst by which catalyst particle attrition is reduced when at least a portion of the oxygen-containing gas required for catalyst fluidization and carbon burn-off is introduced by means of a plurality of gas discharge nozzles into the dense phase of the fluidized catalyst bed.
In a number of high temperature catalytic hydrocarbon processes, e.g., cracking, hydroforming, etc., catalysts which are employed in finely divided or powder form become contaminated with carbonaceous deposits during contact with the hydrocarbon reactants in the conversion zone. These carbonaceous deposits which accumulate on the surfaces of the catalyst particles during the conversion process can, and do, substantially reduce the activity of, or even deactivate, the catalyst. It is common practice, especially in continuous processes for catalytic cracking of heavy hydrocarbons, where the particle size of the spent catalyst admits, to regenerate the catalyst by continuously passing it from the catalytic conversion zone into a regeneration zone where sufficient oxygen-containing gas is introduced under controlled conditions to fluidize the catalyst particles and burn-off a substantial portion of the carbonaceous material deposited thereon. Typically, in such processes the carbon-contaminated catalyst is introduced into the regeneration zone or chamber as a fluidized mass with a portion of the oxygen-containing gas required for regeneration or an inert gas, e.g., nitrogen, being the fluidizing medium or as a solid phase via gravity feed or other suitable mechanical transporting means. In any case, the catalyst particles on entering the regeneration chamber form a dense zone or phase at the lower end of the chamber where catalyst particles and gas interact as a single mass which simulates a liquid and an upper less dense phase in which the catalyst is merely suspended in the gases or vapors. Conventionally, the simulated liquid behavior (fluidized state) of the lower dense phase is imparted and/or maintained by an oxygen-containing gas distribution system comprising one or more manifold members of various shapes, usually substantially horizontally oriented at the bottom portion of the chamber and connected to an outside source of pressurized oxygen-containing gas, each manifold member having a plurality of gas discharge ports through which oxygen-containing gas is introduced at a flow rate sufficient to maintain the fluidized or liquid behavior of the dense catalyst phase. Depending on the amount of oxygen-containing gas utilized to transport the spent catalyst particles into the regeneration chamber, the oxygen-containing gas introduced via gas discharge nozzles into the dense phase of the catalyst bed at the lower end of the regeneration chamber will also supply all or part of the oxygen required to combust a substantial portion of the carbon present on the incoming spent catalyst.
After contacting the oxygen-containing gas at a temperature and residence time sufficient to burn-off a substantial portion of the carbonaceous deposits on the catalyst particles, the catalyst is withdrawn from the regeneration chamber by means of one or more conduits typically located at or near the top of the dense phase and returned to the conversion zone. The spent regeneration gas containing the gaseous combustion products and entrained catalyst particles from the dilute catalyst phase is passed through a conventional solids gas separation scheme, e.g., one or more cyclone separators, to remove the catalyst particles which are returned to the dense phase with the residual gases being discharged to the atmosphere via a waste gas stack.
While the fluidized bed regeneration process described generally above or various modifications thereof have been employed on a broad scale commercially for a number of years such processes are not devoid of problems. One difficulty which continues to plague operators of such regeneration processes, especially in the catalytic cracking area, is the significant attrition of catalyst particles which appears to occur during the regeneration process. The particle size (diameter) of catalysts employed in catalytic cracking typically ranges from about 5 to about 125.mu. (microns) with the major portion of the catalyst particles being in the 45 to 74.mu. range. Catalyst particles in this particle size range can be and are essentially confined in the system as the finer particles, i.e., catalyst particles in the 5-40.mu. range, which remain entrained in the gas phase of the regeneration chamber are effectively recovered by known gas-solids separation techniques, e.g., one or more cyclone (centrifugal) separators located at the top or downstream of the regeneration chamber. The problem lies in the fact that a significant quantity of the catalyst charged to the fluidized regeneration zone is attrited to particles in the less than 5.mu. range under the conditions which exist in conventional regeneration processes. These less than 5.mu. attrition products are too fine to be effectively recovered by conventional cyclonic gas-solids separation systems; and consequently, represent a source of catalyst loss in the system as well as causing atmospheric pollution problems in furnace stacks associated with catalytic cracking units, unless expensive scrubbing or precipitating systems are employed. Accordingly, it would be very desirable from both economic and environmental standpoints if the source of this catalyst attrition could be substantially eliminated at its source, i.e., the regeneration zone itself.